1. Field of the Invention
The present invention relates to a magnetooptical recording medium from which very high-density information can be reproduced by utilizing domain wall displacement during reproduction.
2. Description of the Related Art
Although a magnetooptical disk has recently attracted attention as a reloadable high-density recording medium, it is desired to further increase the recording density of a magnetooptical disk to obtain a large-capacity recording medium. The linear recording density of the magnetooptical disk greatly depends upon the laser wavelength xcex of a reproduction optical system and the numerical aperture NA of an objective lens, and the spatial frequency in signal reproduction has a detectable limit of about NA/xcex.
Therefore, in order to realize a high density by a conventional optical disk, it is necessary to shorten the laser wavelength of the reproduction optical system, and increase the numerical aperture NA of the objective lens. However, improvements in the laser wavelength k and the numerical aperture NA of the objective lens are also limited. Therefore, some techniques have been proposed, in which a recording density is improved by a specific construction of a recording medium, and a reading method.
For example, the applicants proposed in Japanese Patent Laid-Open No. 6-290496 a magnetooptical recording medium in which signals having a period smaller than the light diffraction limit can be reproduced at a high speed without decreasing the reproduction signal amplitude, and a reproduction system and reproducing device therefor. Namely, a temperature distribution is formed in a reproducing layer of the magnetooptical recording medium by heating means such as a light beam or the like to produce a distribution in the domain wall energy density, thereby permitting a domain wall to be instantaneously moved or displaced to the lower domain wall energy side.
As a result, the reproduction signal has a constant maximum amplitude without depending upon the interval of the recorded domain walls (i.e., the recording mark length). In this way, the inevitable decrease in reproduction output accompanying an improvement in the linear recording density is significantly minimized enabling a further increase in density.
The domain wall displacement-type magnetooptical recording medium disclosed by the applicants in the Japanese Patent Laid-Open No. 6-290496 comprises the same recording system (the method of recording on a memory layer) as a conventional magnetooptical recording system.
FIG. 1 schematically shows the result of observation of recording pits (recording magnetic domains 11) from the upper side of a magnetooptical recording medium (disk).
Grooves and lands are formed circumferentially on the magnetooptical recording medium (disk) so that they are arranged alternately in the radius direction of the medium.
In FIG. 1, the recording magnetic domains 11 are formed on each of recording tracks 14. For a reproduction spot 13, a Ts isothermal line 12 is extended elliptically in the movement direction of the medium on the medium. When the leading end of the Ts isothermal line 12 approaches the recording domains 11, domain wall displacement occurs.
In general, as shown in FIG. 1, each of the recording magnetic domains 11 has a circular-arc shape (herringbone shape). This is due to the phenomenon that the Ts isothermal line 12 has a circular shape close to an ellipse during recording, and thus the shape of a recording mark is determined by the circular arc of the isothermal line in a recording track. Here, Ts represents a temperature close to the Curie temperature of a second magnetic layer, as well as a temperature where bonding between first and third magnetic layers is cut, and the domain wall of the first magnetic layer moves to the lower side of the domain wall energy.
In reproduction, domain wall displacement occurs when the Ts isothermal line approaches a recording mark to obtain a reproduction signal in a state where the recording mark is enlarged.
In reproducing a herringbone-shaped recording mark, the recording mark which enters a reproduction spot gradually approaches from the central portion of the herringbone mark in a track, and thus the starting time (timing) of domain wall motion possibly changes with the mark size and the relative speed. Furthermore, the direction of a herringbone is opposite to the direction of the circular arc of the Ts isothermal line in reproduction, thereby causing a problem in that jitter tends to increase.
In order to solve the problem, the inventors found that annealing treatment of both sides of a recording track can change the herringbone shape of a conventional recording mark to a linear shape and is thus effective for decreasing jitter. However, the annealing step causes an increase in medium cost, causing a problem of mass-productivity of media.
The present invention has been achieved in consideration of the above problem, and an object of the present invention is to provide a magnetooptical recording medium suitable for domain wall motion reproduction.
In order to achieve the objects, in accordance with an embodiment of the present invention, a magnetooptical recording medium having domain wall displacement during reproduction comprises a substrate having lands and grooves, and a magnetic layer deposited on the substrate so as to be magnetically interrupted between the lands and grooves, wherein the magnetic film deposited on each of the lands has a section having a flat bottom, and a substantially flat top in the central portion of each of tracks, either side having a rounded upper corner and a thickness decreasing toward the side end.
In accordance with another embodiment of the present invention, a magnetooptical recording medium having domain wall displacement during reproduction comprises a substrate having projections provided between recording tracks, and a magnetic layer deposited on the substrate so as to be magnetically interrupted by the projections, wherein the magnetic layer deposited on each of the recording tracks has a section in which either side has a thickness decreasing toward the side end.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.